Method of drying materials

ABSTRACT

A drying method particularly suited to the drying of pulverulent or granular feed materials to a predetermined residual water content is disclosed. Air is caused to flow past the material being dried from input to exhaust and the temperature of the exhaust air is continuously sensed. In a first stage input air is heated and then maintained at a first temperature for a period of time until after the exhaust air temperature passes a discontinuity and begins to rise rapidly. The first stage is brought to a close when the exhaust air temperature is intermediate the discontinuity temperature and the mean value of the discontinuity temperature and the first mentioned temperature. In a second process stage, the temperature of the input air, which is then preferably humidified, is reduced until the input air temperature and the exhaust air temperature both either asymptotically approach or reach an equilibrium temperature which is defined as the temperature at which the material being dried having the prescribed residual water content is in moisutre transfer equilibrium with the air flowing past it. Optionally, drying may be continued for a period of time with both the input and exhaust air temperatures equal to the equilibrium temperature.

United States Patent Stahl [4 1 Jan. 21, 1975 METHOD OF DRYING MATERIALS Peter Heinrich Stahl, Freiburg im Breisgau, Germany [75] Inventor:

[73] Assignee: Ciba-Geigy Corporation, Ardsley,

22 Filed: Nov. 9, 1972 21 Appl. No.: 304,937

[30] Foreign Application Priority Data Primary ExaminerCharles J. Myhre Assistant ExaminerWilliam C. Anderson Attorney, Agent, or FirmWenderoth, Lind & Ponack [57] ABSTRACT A drying method particularly suited to the drying of pulverulent or granular feed materials to a predetermined residual water content is disclosed. Air is caused to flow past the material being dried from input to exhaust and the temperature of the exhaust air is continuously sensed. In a first stage input air is heated and then maintained at a first temperature for a period of time until after the exhaust air temperature passes a discontinuity and begins to rise rapidly. The first stage is brought to a close when the exhaust air temperature is intermediate the discontinuity temperature and the mean value of the discontinuity temperature and the first mentioned temperature. In a second process stage, the temperature of the input air, which is then preferably humidified, is reduced until the input air temperature and the exhaust air temperature both either asymptotically approach or reach an equilibrium temperature which is defined as the temperature at which the material being dried having the prescribed residual water content is in moisutre transfer equilibrium with the air flowing past it. Optionally, drying may be continued for a period of time with both the input and exhaust air temperatures equal to the equilibrium temperature.

6 Claims, 4 Drawing Figures METHOD OF DRYING MATERIALS FIELD OF THE INVENTION This invention relates to a method of drying a quantity of material to a predetermined residual water content by causing air to flow past the material from input to exhaust.

BACKGROUND TO THE INVENTION In conventional drying methods it has generally been the practice to use hot air of constant humidity and temperature and to determine the drying time by performing a number of experimental tests. These known procedures depend to a substantial degree upon secondary factors, such as the composition of the feed that is to be dried, the final water content of the dry feed, and in granular feeds their grain composition, size and analysis, the size of the treated batch, drying temperatures, throughput of air, dew point of the entering hot air, many of which factors have effects that can be allowed for only empirically. Consequently in such previous drying methods either a desired residual water content cannot be attained at all with any certainty or alternatively the waste of time involved in supplementary drying or damping batches of the dried feed must be accepted as unavoidable. This applies particularly to the drying of freshly granulated materials for which the most suitable drying conditions had first to be found by trial and error when the previous drying procedures were followed.

The previously employed drying methods were considered particularly suitable for drying materials in a fluidised bed although they were by no means limited to such applications. As is well known fluidised solids drying permits very high drying rates to be achieved. However, at such high drying rates it is particularly difficult to keep to a specified residual water content. The drying methods hitherto used were unable to overcome these difficulties satisfactorily.

It is an object of the present invention to provide a drying method which is capable of reliably attaining a prescribed residual water content.

It is another object of this invention to provide a method of drying pulverulent or granular feed materials to a predetermined residual water content.

It is also an object of the present invention to reduce the number of secondary factors which it is required to take into account in performing the method.

These and other objects of the invention will become apparent from the following detailed description when taken together with the accompanying drawings.

SUMMARY OF THE INVENTION The objects of this invention are obtained in a method of drying a quantity of material to a predetermined residual water content by causing air to flow past the quantity of material from input to exhaust, by the steps of:

continuously sensing the temperature of the exhaust air;

heating the input air to a first predetermined temperature;

maintaining the temperature of the input air at said first predetermined temperature for a period of time including a period during which the sensed temperature of the exhaust air rises at a low rate, and expiring after the sensed temperature of the exhaust air begins to rise at an increased rate but before the sensed temperature of the exhaust air reaches the mean value of the temperature from which said increased rate rise began and said first predetermined temperature; and

thereafter, reducing the temperature of the input air until both the input air temperature and the exhaust air temperature asymptotically approach a second predetermined temperature, being the temperature at which said quantity of material with said predetermined residual water content is in moisture transfer equilibrium with the air flowing therepast.

Preferably drying is continued until both the temperature of input air and the temperature of the exhaust air equal said second second predetermined temperature. The input air at the first predetermined temperature preferably comprises dehumidified-air whilst the reduced temperature input air preferably comprises humidified atmospheric air.

Drying methods in accordance with this invention have been found to be particularly suitable for drying pulverulent or granular feed materials, but the methods are equally successful in the drying of grain, shredded fruit, flat materials such as photographic plates or lengths of photographic film, and also porous or other feed materials provided that they are not bulky compact bodies.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 3b is a graph showing the relationship betweendew points and temperature for a given residual water content of a feed material.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring again to FIG. 1 there is shown schematically an embodiment of a fluidised bed dryer adapted for performing a drying method in accordance with this invention. The dryer comprises a chamber 1, in which is provided a grating 2 for supporting a quantity of pulverulent or granular feed 3 to be dried. A fan 4 is adapted to induce an air draught through the chamber to cause a flow of air through the bed of feed material 3, the air entering through an inlet 5 and leaving through an exhaust outlet 6. As shown a filter 7, a cool ing grid 8 and a heating grid 9 are provided between the inlet 5 and the grating 2. Grating 2 defines one side of a chamber 10 for the feed 3 being dried, the other side being defined by a filler II. An indicating instrument 12 is connected to a measuring head 13 provided in the path of intake air for determining the dew point 1 of the intake air. A temperature measuring means 14 is provided immediately upstream of grating 2 for the determination of the temperature 0, of the intake air entering the fluidised bed; and a temperature measuring means 15 is provided immediately downstream of filter 11 for the determination of the temperature 0,, of the out-flowing exhaust air.

The temperature measuring elements 14 and 15 are both associated with a recorder 16 and also with respect controllers l8 and 17. Controller 17 is connected to controller 18 which is in turn connected by leads 19 and 20 to valves 21, 22 controlling the cooling grid 8 and the heating grid 9 respectively.

As will be best understood by reference to FIG. 2, the drying method proceeds in two main stages which may optionally be followed by a third stage. In the first stage the inflowing input air is first heated to a predetermined temperature O the exact value of which is not critical. The dew point r, of the input air is determined by measuring head 13 and instrument 12. By reference to the dew point-temperature curve of the feed for the desired residual water content an equilibrium temperature 0 for the measured value 1', of the input air dew point can be obtained.

The temperature of the exhaust air is kept continuously under observation. The initial adjusted temperature 6, of the input air is maintained until the exhaust air temperature, which initially rises slowly passes a discontinuity at a temperature O and thereafter begins to rise rapidly. At a point in time chosen to be during the interval in which the exhaust temperature is intermediate B and (6 0,)l2, the drying process enters a second stage. In other words, the first drying stage is continued until after the sensed exhaust air temperature has passed the discontinuity and begun to rise rapidly but is stopped before the sensed exhaust temperature has reached the mean of the temperature from which the rise began and the predetermined input air temperature.

From this point in time onwards the input air temperature is reduced so that both the input air temperature 0, and the exhaust air temperature 0,, either asymptotically approach or reach the previously found temperature 6 When 6 0,, 0,, operation of the dryer is preferably continued for a while under these conditions. At the end of this third drying stage which may constitute onefifth to one-tenth of the total time of the process, drying is discontinued. In order to reduce the overall time, particularly when the input air is very dry and consequently the equilibrium temperature would be low, it is advantageous to humidify the input air during the second and third stages of the drying process. The equilibrium temperature 0,, for the humidified input air is that which is then required.

For determining the equilibrium temperature the dew point-temperature curve of the feed for the desired residual water content is required. This can be obtained by any method known in the art. By way of example, the dew point was measured directly at a rising sequence of sampling temperatures immediately above a specimen of the feed which possessed the desired water content. The curve obtained by plotting temperature 0 against dew point 'r is shown in FIG. 3b for the desired water content.

In an alternative procedure the dew pointtemperature curve is obtained from the water vapour desorption isotherms, which are first obtained experimentally for the feed concerned. From the desorption isotherms of the feed the equilibrium conditions of humid air are taken for the desired residual water con- 6 above described, are used for plotting a dew pointtemperature curve (FIGS. 3a and 3b).

Should the feed that is to be dried consist of two or more components having desorption isotherms that are known, then the desorption isotherms of the feed can be approximately calculated from the desorption isotherms of the components by adding the contribution of the several components to the total water content of the feed for every point of the isotherm, in accordance with the relative proportions of the several components in the feed. The calculated desorption isotherms thus obtained are treated in the same manner as before to produce a corresponding dew point-temperature curve from which the equilbrium temperature 0 can be read off.

Since the steps of the drying method are controlled by properties of the product, the dimensions of the dryer, the size of the batch, the final humidity, the mean grain size, the grain size analysis and grain structure have no effect on the overall result of the drying process. These parameters merely affect the duration of the second drying phase. The residual humidity which remains in the feed is evenly distributed throughout the cross section of the individual grain and any risk of the fines excessively drying out is substantially less than in previous drying methods. Since the first phase is not tied to a particular drying temperature optimum adaptation to the temperature stability of the feed can be achieved. Since the humidity of the exhaust air need not be measured, the provision of expensive and rugged hygrometers is unnecessary. Instead, only the input air humidity which is much easier to measure is required to be determined. Consequently the method described is very suitable for automation.

What is claimed is:

1. In a method of drying pulverulent or granular feed materials to a predetermined residual water content by causing air to flow past said feed material from input to exhaust, the steps of:

continuously sensing the temperature of the exhaust air;

heating the input air to a first predetermined temperature prior to its introduction into said feed mate rial;

maintaining the temperature of the input air at said first predetermined temperature for a period of time including a period during which the sensed temperature of the exhaust air rises at a low rate, and expiring after the sensed temperature of the exhaust air begins to rise at an increased rate but before the sensed temperature of the exhaust air reaches the mean value of the temperature from which said increased rate rise began and said first predetermined temperature; and

thereafter, reducing the heating temperature of the input air until both the input air temperature and the exhaust air temperature asymptotically approach a second predetermined temperature, being the temperature at which said feed material with said predetermined residual water content is in moisture transfer equilibrium with the air flowing therepast and no further exchange of moisture between said feed material and air occurs.

2. A drying method according to claim 1, wherein drying is continued until both the temperature of the input air and the temperature of the exhaust air equal said second predetermined temperature.

3. In a method of drying a pulverulent or granular feed material to a predetermined residual water content by causing air to flow past said feed material from input to exhaust, the steps of;

continuously sensing the temperature of the exhaust air;

heating the input air to a first predetermined temperature prior to its introduction into said feed material;

maintaining the temperature of the input air at said first predetermined temperature for a period of time including a period during which the sensed temperature of the exhaust air rises at a low rate, and expiring after the sensed temperature of the exhaust air begins to rise at an increased rate but before the sensed temperature of the exhaust air reaches the mean value of the temperature from which said increased rate rise began and said first predetermined temperature; and

thereafter, reducing the heating temperature of the input air until both the input air temperature and the exhaust air temperature reach a second predetermined temperature, being the temperature at which said feed material with said predeterined residual water content is in moisture transfer equilibrium with the air flowing therepast and no further exchange of moisture between said feed material and air occurs.

4. A drying method according to claim 3, wherein the reduced temperature input air comprises humidified atmospheric air.

5. A drying method according to claim 3, wherein the input air at said first predetermined temperature comprises dehumidified air and the reduced temperature input air comprises humidified atmospheric air.

6. A method of drying pulverulent or granular feed materials to a predetermined residual water content.

the method comprising:

providing a drying chamber having an inlet and an outlet;

supporting a bed of said feed material intermediate said inlet and said outlet;

continuously sensing the temperature of air passing to said outlet;

causing dehumidified air to flow through said bed of material from said inlet to said outlet;

heating the dehumidified inlet air to a first predetermined temperature prior to its introduction into said feed material;

maintaining the temperature of the dehumidified inlet air at said first predetermined temperature for a period of time including a period during which the sensed temperature of air passing to the outlet rises at a low rate, and expiring after the sensed temperature of the air passing to said outlet begins to rise at an increased rate but before the sensed temperature of air passing to said outlet reaches the mean value of the temperature from which said increased rate rise began and said first predetermined temperature; and

thereafter causing humidified air to pass through said bed of material from said inlet to said outlet and reducing the heating temperature of the humidified inlet air until both the humidified inlet air temperature and the temperature of air passing to the outlet asymptotically approach a second predetermined temperature, being the temperature at which said feed material with said predetermined residual water content is in moisture transfer equilibrium with the humidified air flowing therepast and no further exchange of moisture between said feed material and said air occurs. 

1. In a method of drying pulverulent or granular feed materials to a predetermined residual water content by causing air to flow past said feed material from input to exhaust, the steps of: continuously sensing the temperature of the exhaust air; heating the input air to a first predetermined temperature prior to its introduction into said feed material; maintaining the temperature of the input air at said first predetermined temperature for a period of time including a period during which the sensed temperature of the exhaust air rises at a low rate, and expiring after the sensed temperature of the exhaust air begins to rise at an increased rate but before the sensed temperature of the exhaust air reaches the mean value of the temperature from which said increased rate rise began and said first predetermined temperature; and thereafter, reducing the heating temperature of the input air until both the input air temperature and the exhaust air temperature asymptotically approach a second predetermined temperature, being the temperature at which said feed material with said predetermined residual water content is in moisture transfer equilibrium with the air flowing therepast and no further exchange of moisture between said feed material and air occurs.
 2. A drying method according to claim 1, wherein drying is continued until both the temperature of the input air and the temperature of the exhaust air equal said second predetermined temperature.
 3. In a method of drying a pulverulent or granular feed material to a predetermined residual water content by causing air to flow past said feed material from input to exhaust, the steps of; continuously sensing the temperature of the exhaust air; heating the input air to a first predetermined temperature prior to its introduction into said feed material; maintaining the temperature of the input air at said first predetermined temperature for a period of time including a period during which the sensed temperature of the exhaust air rises at a low rate, and expiring after the sensed temperature of the exhaust air begins to rise at an increased rate but before the sensed temperature of the exhaust air reaches the mean value of the temperature from which said increased rate rise began and said first predetermined temperature; and thereafter, reducing the heating temperature of the input air until both the input air temperature and the exhaust air temperature reach a second predetermined temperature, being the temperature at which said feed material with said predeteRined residual water content is in moisture transfer equilibrium with the air flowing therepast and no further exchange of moisture between said feed material and air occurs.
 4. A drying method according to claim 3, wherein the reduced temperature input air comprises humidified atmospheric air.
 5. A drying method according to claim 3, wherein the input air at said first predetermined temperature comprises dehumidified air and the reduced temperature input air comprises humidified atmospheric air.
 6. A method of drying pulverulent or granular feed materials to a predetermined residual water content, the method comprising: providing a drying chamber having an inlet and an outlet; supporting a bed of said feed material intermediate said inlet and said outlet; continuously sensing the temperature of air passing to said outlet; causing dehumidified air to flow through said bed of material from said inlet to said outlet; heating the dehumidified inlet air to a first predetermined temperature prior to its introduction into said feed material; maintaining the temperature of the dehumidified inlet air at said first predetermined temperature for a period of time including a period during which the sensed temperature of air passing to the outlet rises at a low rate, and expiring after the sensed temperature of the air passing to said outlet begins to rise at an increased rate but before the sensed temperature of air passing to said outlet reaches the mean value of the temperature from which said increased rate rise began and said first predetermined temperature; and thereafter causing humidified air to pass through said bed of material from said inlet to said outlet and reducing the heating temperature of the humidified inlet air until both the humidified inlet air temperature and the temperature of air passing to the outlet asymptotically approach a second predetermined temperature, being the temperature at which said feed material with said predetermined residual water content is in moisture transfer equilibrium with the humidified air flowing therepast and no further exchange of moisture between said feed material and said air occurs. 